Drop ErrorView's widget prop in favour of the host bridge

ErrorView needed a widget only to decide whether to offer a close button
or a link home. That prop was threaded down through ErrorPage, RichError
and GroupCallErrorBoundary from seven call sites, to answer the single
question of whether the host can dismiss Element Call — which the host
bridge now answers directly through the presence of close().

Read the bridge from context in ErrorView and remove the prop, along with
the plumbing that carried it.

No functional change: the rendered output is unchanged, as the existing
snapshot confirms.

Move GroupCallView onto the host bridge

GroupCallView asked the widget API to keep it on screen, to close it, and
to tell it when a preloaded call should join. Route all three through the
host bridge and drop the widget prop.

src/useTheme.ts

@@ -6,39 +6,27 @@ Please see LICENSE in the repository root for full details.
 */
 
 import { useEffect, useLayoutEffect, useRef, useState } from "react";
-import { WidgetApiToWidgetAction } from "matrix-widget-api";
-import { type IThemeChangeActionRequest } from "matrix-widget-api";
 
 import { useUrlParams } from "./UrlParams";
-import { widget } from "./widget";
 import { useRootElement } from "./RootElementContext";
+import { useHostBridge } from "./HostBridge";
 
 export const useTheme = (): void => {
   const rootElement = useRootElement();
+  const hostBridge = useHostBridge();
   const { theme } = useUrlParams();
   const [requestedTheme, setRequestedTheme] = useState(theme);
   const previousTheme = useRef<string | null>(rootElement.classList.item(0));
 
   useEffect(() => {
-    if (widget) {
-      const onThemeChange = (
-        ev: CustomEvent<IThemeChangeActionRequest>,
-      ): void => {
-        ev.preventDefault();
-        if ("name" in ev.detail.data && typeof ev.detail.data.name === "string")
-          setRequestedTheme(ev.detail.data.name);
-        widget!.api.transport.reply(ev.detail, {});
-      };
-
-      widget.lazyActions.on(WidgetApiToWidgetAction.ThemeChange, onThemeChange);
-      return (): void => {
-        widget!.lazyActions.off(
-          WidgetApiToWidgetAction.ThemeChange,
-          onThemeChange,
-        );
-      };
-    }
-  }, []);
+    const subscription = hostBridge.themeChange$.subscribe(
+      ({ data, reply }) => {
+        if (typeof data.name === "string") setRequestedTheme(data.name);
+        reply();
+      },
+    );
+    return (): void => subscription.unsubscribe();
+  }, [hostBridge]);
 
   useLayoutEffect(() => {
     // If no theme has been explicitly requested we default to dark